A typical Synchronous Optical Network (SONET) ring transmission system for carrying telecommunications traffic includes a plurality of nodes at which telecommunications traffic may originate and terminate. In practice, each node takes the form of a fiber-optic hub and associated cross-connect system for coupling to a telephone switch that sends traffic to, and receives traffic from, the node. Every node is linked to each of a pair of neighboring nodes in daisy-chain fashion by at least one optical fiber link to yield ring-like structure across which telecommunications traffic may pass. When the nodes are separated from each other by large distances, it is not desirable from a cost standpoint to connect all of the nodes in a single ring. Rather, the nodes that are relatively closely spaced are connected in individual rings having at least one node connected to a node in another ring. Alternatively, two or more rings may effectively share nodes to permit traffic to pass from one ring to another.
If a single fiber optic link in a ring should fail, the traffic that would otherwise pass over the failed link can usually be re-routed about the remainder of the ring between an origin and node (where traffic originates) and a destination node (at which traffic terminates). Should two links fail in the same ring of a multi-ring system, it has been necessary in the past to physically repair at least one of the failed links in order to restore traffic on the ring. Such physical repairs are often time consuming. While such repairs are being made, traffic remains disrupted.
Thus, there is a need for a technique for restoring traffic in a multiple ring system via alternate rings when two or more links in one ring have failed.